lavf/hlsenc: Do not mix declarations and code.

Fixes the following warnings:
libavformat/hlsenc.c: In function 'hls_write_trailer':
libavformat/hlsenc.c:2364:17: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
                 uint8_t *buffer = NULL;
                 ^~~~~~~
libavformat/hlsenc.c:2372:17: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
                 int byterange_mode = (hls->flags & HLS_SINGLE_FILE) || (hls->max_seg_size > 0);
                 ^~~
libavformat/hlsenc.c:2379:13: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
             int range_length = 0;
             ^~~

(cherry picked from commit fc94e970)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>